The present invention relates to an apparatus for forming a hollow article in which devices such as a die shifter, die compacting device and air blowing nozzle are electrically driven to operate automatically under a predetermined program, thus achieving improvement in the quality of the hollow article as the product.
Hitherto, apparatus for forming hollow articles employed hydraulic or pneumatic driving systems as shown in FIGS. 1 and 2.
Each of these known driving systems includes a bed 1, a base 2 situated on the bed 1, and an extruder 3 carried by the base 2. The extruder 3 has a hopper 4, a cylinder 6 receiving a screw 5, and a crosshead 7. An air blowing device 8 fixed to a frame 9 carried by the bed 1 is disposed adjacent to the extruder 3.
The air blowing device 8 has an air blowing nozzle 10 which is adapted to move up and down as indicated by an arrow A by means of a blowing device driving hydraulic cylinder 8A. A die 13 is disposed under the crosshead 7 and the air blowing device 8. The die 13 is adapted to be moved in the direction of an arrow B by means of a die shifting device 12 which is actuated by a die shifting hydraulic cylinder 11. The die 13 is so constructed as to be opened and closed by a compacting device 15 having a hydraulic cylinder 14.
The blowing device driving hydraulic cylinder 8A, die shifting hydraulic cylinder 11 and the compacting hydraulic cylinder 14 are supplied with hydraulic oil from a hydraulic unit 16 which is mounted in the base 2. A parison cutting device 2 is actuated by a pneumatic cylinder 20 provided in the frame 9 so as to cut the parison 22.
The oil displaced from the hydraulic unit 16 is supplied to the compacting hydraulic cylinder 14 of the die compacting device 15 so as to open and close the die 13. The oil also is supplied to the die shifting hydraulic cylinder of the die shifting device 12 so as to effect shifting of the die 13. The oil also is supplied to the blowing device driving hydraulic cylinder 8A so as to drive the air blowing nozzle 10 up and down.
In operation, a molten resin extruded from the cylinder 6 is introduced to a crosshead 7 so as to form a cylindrical parison 22 suspended from the crosshead 7. The cylindrical parison 22 is introduced into the die 13. When the length of the parison 22 has reached a predetermined value, the die 13 is closed to clamp the parison 22. Then, the parison 22 is cut at its upper portion by the parison cutting device 21 while the die 13 is shifted obliquely downward by the operation of the die shifting device 12.
Subsequently, the air blowing nozzle 10 is inserted into the parison 22 to blow air into the parison 22, whereby the parison 22 is inflated in conformity with the internal configuration of the die 13. The molten resin forming the parison 22 is then cooled and solidified, whereby the desired hollow article is obtained.
The known method and apparatus for forming a hollow article have suffered from the following problems due to the construction described above.
The pressurized oil tends to leak from the hydraulic unit, hydraulic cylinder and piping so as to contaminate the formed product. In particular, leak of oil from the air blowing device hydraulic cylinder causes a serious defect because such leaking oil may enter the hollow article product such as a bottle which is disposed under this hydraulic cylinder.
The temperature of the oil is low in the beginning of the operation but is gradually raised, which varies the viscosity of the oil as the apparatus over a operates long period of time. This change in the viscosity causes a change in the displacement of the oil from the oil unit to vary the speed of operation of the hydraulic cylinder and the position at which the hydraulic cylinder is stopped, with the result that the quality of the product fluctuates of defective articles are produced.
Stabilization of the oil temperature requires warm-up operation which takes a considerably long time. In addition, the hydraulic unit must be equipped with a specific oil temperature control device which raises the cost of the whole apparatus.
Furthermore, the pump/motor of the hydraulic unit continuously operates even when none of the hydraulic cylinders is operating, thus wasting energy.
Furthermore, energy is wastefully consumed due to pressure drop or resistance to the flow of the oil along the piping which interconnects the hydraulic unit and each hydraulic cylinder. It is also necessary to supply a cooler or an oil tank with cooling water in order to regulate the temperature of the oil which otherwise will rise during the operation. Consequently, the cost of operating the apparatus is considerably increased due to use of cooling water at a large rate.
The forming operation may require that the die be stopped at a position intermediate the full open and full closed positions. In such a case, the die may fail to stop at the desired position or may continue to move slowly beyond the desired position, due to internal leakage of the oil inside the hydraulic cylinder or solenoid valve or due to the influence of back pressure in the hydraulic cylinder. This causes a critical problem from the viewpoint of safety, as well as production of defective products.
The velocity of movement of the cutting blade of the parison cutting device is determined by the rate of supply of the air supplied to the pneumatic cylinder and, hence, tends to vary during the operation due to a variation in the air source pressure. This also leads to production of defective articles. The rate of supply of the air is controllable manually by varying the degree of opening of an orifice valve. This control relies upon experience and perception of the operator and, therefore, tends to vary according to individual operators. It has therefore been difficult to obtain a high degree of reproducibility of production conditions. The conventional parison cutting device which is operated pneumatically tends to be affected by disturbance due to the compressible nature of air. Driving of the cutting device by a hydraulic actuator also leads to unstable operation due to change in the oil viscosity caused by a change in the oil temperature. Simultaneous use of pneumatic and hydraulic actuators cannot provide fine or delicate control of the cutting device, so that the control is coarse, which impairs the reliability of operation.
A problem also is caused due to the fact that the parison is cut while it is being drawn obliquely between the stationary crosshead and the obliquely die. The problem resides in that the cut parison does not exhibit exact cylindrical form with a horizontal cut edge but, instead the inner side of the parison tends to be collapsed. Consequently, the end of the air blowing nozzle forcibly displaces the parison when the nozzle is inserted into the parison. This makes it difficult to obtain a product such as a bottle having the expected configuration of the neck, thus lowering the product yield from the apparatus.